A possibility is secretion from a gland that lubricates the urethra and balances the phin preparation for ejaculation. One doesn't necessarily need to get an erection for this gland to start producing fluid, but just thinking sexual thoughts or looking at a Playboy magazine...

Also, the prostate continually produces fluid,and if its been a long while since the last ejaculation, fluids from this gland may back up into the bladder. During urination the urine will leave first, and then this fluid can slowly leak out. (so common among teenagers, who have a never ending amount of sexual energy)

Hopefully there's no problem. I'm guessing one of the two instances above.